We have investigated the physical mechanism of off-state breakdown in GaAs MESFETs that exhibited an initial breakdown voltage shift called "walkout" and snapback in I-V characteristics. From experiments using dual-gate MESFETs under various bias stress conditions, we attributed the origin of breakdown walkout to the change in electrical properties of the surface state at the gate edge. This was confirmed by experiments using newly developed open-gate FETs whose surface state was changed from an electron trap to a hole trap in the ungated region within about 0.4 mum from the gate edges. The change in surface-state properties can be explained by assuming electron injection from the gate metal into the oxide layer and the following surface Fermi-level dynamics. To verify our breakdown walkout model, we performed a two-dimensional simulation of gate-drain breakdown in GaAs MESFETs taking into account impact ionization, tunneling, and the proposed surface-state model. Our simulation can successfully describe the experimentally observed breakdown behavior, i.e., walkout and snapback. (C) 2002 Elsevier Science Ltd. All rights reserved.